2.0 Analysis 2.1 Introduction After experiencing a hydraulic failure, the pilot flew the helicopter for approximately three minutes while transiting to the Mekatina logging site. In the absence of an ideal landing site, he flew a low reconnaissance over the area with the intention of conducting a landing in a somewhat confined area. The helicopter crashed after departing controlled flight while manoeuvring for the landing. This analysis will focus on the reason for the hydraulic system failure, the flight characteristics of the helicopter when hydraulics are lost, and the pilot's actions following the hydraulic failure. A hydraulic system failure in the AS350B2 helicopter is an emergency requiring prompt action by the pilot. The helicopter is designed to revert safely to manual controls and should be controllable to a safe landing if proper procedures are followed. However, in this case, the fact that the hydraulicCB was likely out, rendering the CUTOFF switch inoperative, would have changed the manner in which the flight control system reverted to manual controls and may have resulted in asymmetric depletion of the accumulator pressures. This may have resulted in the pilot being uncertain of the status of the hydraulic system and flight controls. 2.2 Hydraulic System Failure The hydraulic failure was the initiating event that led to the accident. It is likely that the hydraulic pump drive belt failed in flight and precipitated the hydraulic failure. This is supported by the weakened pre-impact condition of the failed belt, the deteriorated condition of the other in-service belts that were examined, and the history of similar failures. No other explanation for a hydraulic failure was found. It is also likely that the hydraulicCB was in the tripped position in flight for undetermined reasons. The investigation of this accident has failed to reveal any other component failures in the hydraulic system that would explain why control of the aircraft was lost. Laboratory testing of the hydraulic pump drive belt yielded some significant results. The extensive cracking in the same location in all the comparison samples (except for the new belt) indicates that a design problem may exist at that location, creating a stress/strain concentration that results in a consistent and predictable failure. The applied stresses and strains may exceed the design's strength or the service life of the belt may be too long. A review of the TC Service Difficulty Report database revealed numerous drive-belt failures that occurred during the belt in-service life of 600hours. Since belts failed as early as 80hours, reducing the service life of the belt is unlikely to significantly reduce belt failures. Similarly, a visual inspection of the installed drive belt is not likely to reveal cracking or weakening at the seam. In order to have a reasonable expectation of detecting cracks with a visual inspection, the drive belt must be removed, turned inside-out, put under some tension and carefully inspected. As a result of the serviceability issues with the original belt, on 27May2002, Eurocopter issued Service Bulletin (SB) No.63.00.08, offering an improved (Poly-V) belt with a significantly longer service life (1500hours). Based on an optionalSB, discussions with the manufacturer's technical representative on the unproven reliability of the new Poly-V belt, MNR's infrequency of belt failures and its self-imposed reduction in the replacement time of the belt, MNR chose not to replace its belts. Prior to this accident, MNR had only one belt failure in over 10years of operation. Standard practice for MNR was to reduce the manufacturer's recommended belt service life and replace their hydraulic system belts at 500hours, rather than the manufacturer's limit of 600hours. TC subsequently issued an Airworthiness Directive (AD) on 22April2004 (No.CF-2004-10), mandating the new Poly-V belt modification. It could not be determined if the pilot selected the hydraulic CUTOFF switch to CUTOFF. If he did and the hydraulic CB was tripped before he made the selection, the switch would not have functioned. In that case, the main servo solenoid valves would not be activated and the servo's pressure inlet to the return line would not be opened. The residual pressure in the main servo accumulators may not be depleted evenly, and a smooth transition to manual controls may not occur. At some point prior to the crash, the pilot moved the HYDTEST switch to the Test position. This is not recommended in flight and would normally result in the loss of hydraulic pressure in the tail-rotor servo system. However, it appears that the hydraulicCB was already tripped when the HYDTEST switch was moved. This is supported by the presence of hydraulic pressure in the tail-rotor servo accumulator after the occurrence; this pressure would have provided for tail-rotor yaw control. 2.3 Pilot's Actions According to his training and the AFM emergency procedures, when confronted with the hydraulics failure, the pilot would be expected to slow the helicopter to the recommended speed range (40-60knots) and conduct a flat approach over a clear landing area, and land with slight forward speed. Since he was confronted with an abnormal situation in which emergency response actions did not result in predictable results, the pilot may have elected to fly the helicopter at higher airspeeds in order to reach the Mekatina landing site sooner. His decision on where to land may have been influenced by the depth of the snow on nearby Hion Lake, the availability of personnel at the Mekatina logging site, and the fact that the logging site was accessible by road. As a result of not slowing the helicopter to the recommended speed, the pilot would have detected higher control forces once the accumulators on the main-rotor servos were depleted. The positions of the hydraulic HYDTEST switch and the hydraulic CUTOFF switch, as found at the occurrence site, indicated that the pilot may have attempted to dump the hydraulic pressure in flight by use of the HYD TEST switch, an inappropriate method that is not in accordance with the AFM. However, given that the pilot was confronted with an abnormal emergency situation due to the tripped hydraulicCB, it is possible that he selected the HYDTEST switch when he recognized that the CUTOFF switch did not function. There is no indication that this action further exacerbated the hydraulic emergency. As it is likely that the hydraulicCB was tripped in flight, the hydraulic pressure from the main-rotor servos was likely depleted asymmetrically, thereby presenting the pilot with uneven cyclic loads. The experienced pilot would have been flying with a firm grip on the controls in anticipation of increased control loads associated with hydraulic pressure depletion. It is unlikely that an approximate 5daN (deca-newton) or 11-pound sudden and short-duration increase in asymmetric cyclic load to a pilot anticipating a load increase could result in the pilot losing control of the helicopter. He may have attempted to dump the hydraulic pressure using the HYDTEST switch after realizing that the hydraulic CUTOFF switch had no effect. While manoeuvring to land at the logging site, the aircraft was seen to enter a left turn from which it did not recover. The forces encountered by the pilot during that turn at low altitude may have been too extreme to overcome, making it impossible for him to recover the aircraft to level flight. 3.0 Conclusions 3.1 Findings as to Causes and Contributing Factors After experiencing a hydraulic system failure, the helicopter departed controlled flight and crashed while manoeuvring for landing. The reason for the departure from controlled flight could not be determined. It is likely that the hydraulic pump drive belt failed in flight, precipitating the hydraulic failure. It is likely that the hydraulic circuit breaker was in the tripped position in flight, rendering the hydraulic CUTOFF and HYDTEST switches inoperative. This would result in hydraulic pressure from the main-rotor servos being depleted asymmetrically. 3.2 Findings as to Risk Laboratory examination of the failed hydraulic drive belt and other similar unbroken belts from other aircraft revealed extensive cracking in the same location in all the comparison samples. A problem may exist at that location, creating a stress/strain concentration that results in a consistent and predictable failure. 3.3 Other Findings The forces encountered by the pilot during the turn at low altitude may have been too extreme to overcome, making it impossible for him to recover the aircraft to level flight. The disassembly and/or examination of the four hydraulic servo controls and the components of the main-rotor controls revealed no pre-existing condition that would have prevented normal operation. Hydraulic fluid test results identified a water content that was within the maximum allowable limit. 4.0 Safety Action 4.1 Action Taken Based on the occurrence involving Remote Helicopters and this MNR accident, and referencing the initial data gathered by the TSB/Transport Canada (TC) working group, TC issued an Urgent Airworthiness Directive (AD), CF-2003-15, dated 16May2003. The AD, Eurocopter (Arospatiale) AS350 - Loss of Hydraulic Power, stated the following: All AS350 operators inform all flight crew to perform a thorough pre-flight check and ensure that the accumulator hydraulic pressure is depleted and verified for any unusual force or movement of the controls. Investigate the cause of uncommanded movement thoroughly prior to return to service. Any irregularities need to be reported to Continuing Airworthiness Branch, Transport Canada. The aircraft be landed as soon as possible after a hydraulics failure to mitigate extended utilization in manual mode. Flight with hydraulics off for non-emergency situations is curtailed. The AD was superseded by ADCF-2003-15R1, issued 01July2003, which stated that the pre-flight check is to be conducted prior to EVERY flight. On 23September2003, TC issued Airworthiness Notice (AN) D006, Edition1, the purpose of which was to update aircraft owners/operators, maintainers and pilots of AS350 rotorcraft on the TSB investigation. Specifically, it addressed concerns expressed with the flight control characteristics of the AS350 when the hydraulic system pressure is lost. One anomaly addressed in the AN is stated as follows: Uncommanded movement when system is being depleted: Eurocopter ground test demonstrated that an uncommanded movement is possible when one lateral accumulator is depleted and the other charged. Based on this test, Transport Canada concluded that a similar result may be expected in flight. The above uncommanded input is prevented in-flight when the pilot follows the Aircraft Flight Manual (AFM) procedure, which indicates that following a hydraulic failure, the aircraft is slowed promptly to a specified speed and the hydraulic cut-off is activated. By activating the cut-off, any unbalanced force as a result of asymmetrical residual accumulator pressure is avoided. If the hydraulic cut-off is not activated, then the sustained asymmetrical pressure may occur if the accumulator depletes at a different rate until the residual pressure is depleted through normal movement of the flight controls. The force identified by the Eurocopter test would be approximately 5daN (11pounds). The AN states in part that TC, in cooperation with the Direction Gnrale de l'Aviation Civile (DGAC), has undertaken to continue to examine the servos that have been removed as a result of uncommanded movement to ensure that there are no hidden failure modes. Also, TC intends to conduct a review of all flight control failure modes related to the control systems and associated in-flight handling qualities in various flight regimes and environmental conditions.